People can barely agree how to store dates independently of JSON, choosing one would be add extra work for implementers, probably not be used by people who prefer other date formats, make JSON un-eval-able, fragment implementations between those that support dates and those that not, and hurt it's reputation as a consistent, simple format.
is eval-able, though of course Javascript is hampered by having standardized a clumsy API that predates Joda-Time (e.g., days are one-based but months are zero-based) and it's far too late to get JSON implementations to support any constructor or function calls.
Also odd that the spec only has the railroad diagrams from json.org, when any implementor would find the BNF grammar more code-ready and probably easier to follow.
